Electrode arrays for insertion into the cochlea are known in various forms in the prior art. They are generally manufactured in a straight form from a resilient material. When they are inserted into the cochlea, they flexibly curve into the spiral form of the scala tympani. However, the electrode array is resilient and hence tends to "remember" its straight form, and accordingly engages the outer (radially) wall of the scala tympani. For optimum electrical stimulation to occur, it is preferred that the electrodes engage the inner wall, near the modiolus.
One solution which has been proposed is to manufacture the electrode in a spiral shape. This type of arrangement is shown in U.S. Pat. No. 4,284,085 to Hansen et al, and in the device developed by the University of California at San Francisco. However, these devices are difficult to insert in a surgical procedure, and require specialised equipment and skills to approach satisfactory performance. Moreover, they use a curve for the array which is an estimate of average shape, not the actual shape of each patient's cochlea. These devices also require very careful manufacturing techniques to produce a reliable product.